media-tv/v4l-utils: Only check Clang in pkg_pretend when installed

I forgot that pkg_pretend() fires before dependencies are
installed. We therefore need to check again in pkg_setup().

Bug: https://bugs.gentoo.org/763654
Package-Manager: Portage-3.0.13, Repoman-3.0.1
Signed-off-by: James Le Cuirot <chewi@gentoo.org>

media-tv/v4l-utils/v4l-utils-1.20.0.ebuild

@@ -46,7 +46,7 @@ PATCHES=(
 # Not really prebuilt but BPF objects make our QA checks go crazy.
 QA_PREBUILT="*/rc_keymaps/protocols/*.o"
 
-pkg_pretend() {
+check_llvm() {
 	if [[ ${MERGE_TYPE} != binary ]] && use bpf; then
 		local clang=${ac_cv_prog_CLANG:-${CLANG:-clang}}
 		${clang} -target bpf -print-supported-cpus &>/dev/null ||
@@ -54,6 +54,14 @@ pkg_pretend() {
 	fi
 }
 
+pkg_pretend() {
+	has_version -b sys-devel/clang && check_llvm
+}
+
+pkg_setup() {
+	check_llvm
+}
+
 src_prepare() {
 	default
 	eautoreconf